The invention relates to a process and a circuit configuration for converting a frequency signal to a DC voltage.
To convert a frequency signal to an electrical DC voltage it has become known, for example, from UKW-Berichte 2/88, pp. 98-105, to use a monostable trigger circuit (univibrator, monostable multivibrator, or monoflop). Upon a trigger event, such a monostable trigger circuit supplies as an output signal a one-time rectangular pulse, the width or time duration T0 of which can be predefined by an external RC element. If a frequency signal, i.e., a voltage signal periodically oscillating between two values, such as a periodic rectangular signal of frequency f=1/T, is applied to the trigger input of the monostable trigger circuit, then rectangular pulses of the duration To and a period T are created at the output of the monostable trigger circuit as long as the frequency of the frequency signal is less than 1/T0.
A lowpass filter is connected in series with the output of the monostable trigger circuit. The cutoff frequency fg of the lowpass filter is much lower than frequency f of the frequency signal. A DC voltage signal is obtained at the output of the lowpass filter, the magnitude of which corresponds to the average time value of the rectangular pulse sequence present at the output of the monostable trigger circuit. Since the pulse duration T0 of the rectangular pulses is constant, the magnitude of the DC voltage signal is proportional to the frequency of the frequency signal.
Such a circuit configuration, which is, known as a frequency-to-voltage converter, can be used, for example, as a tachometer generator (tacho-alternator) or as an EM demodulator. Preferably, such a circuit configuration is used for frequency stabilization of tunable oscillators.
In has been shown in practical applications that the magnitude or the value of the DC voltage signal is not exactly proportional to the frequency of the frequency signal. In other words, the relationship between the magnitude of the DC voltage signal and the frequency is not precisely linear. This is caused, for example, by transfer processes that have not been fully completed so that with increasing frequency of the frequency signal the magnitude of the DC voltage signal is always slightly smaller than it would have to be in the case of exact linearity. In a tunable oscillator, for example, this causes frequency deviations or reading errors on the generally linear scale.
Frequency-to-voltage converter ICs with relatively good linearity are known, such as IC AD650 by Analog Devices. These devices, however, require high internal circuit complexity and are therefore substantially more expensive than standard monoflops of the digital CMOS or TTL series. The price difference may be as high as a factor of 100. Furthermore, those known frequency-to-voltage converter ICs are suitable only for an input frequency of up to approximately 1 MHz. In contrast, a conventional TTL standard monoflop of Type SN74123, for example, can generate rectangular pulses with a minimum duration T0 of up to about 40 ns at the output. Thus, in principle, it would be possible with this standard monoflop to process frequency signals of up to about 20 MHz.
It is accordingly an object of the invention to provide a process for converting a frequency signal to a DC voltage, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which permits, with little circuit complexity, an improved linear correlation between the frequency of the frequency signal and the value of the DC voltage signal even when simple standard monoflops are used. It is a further object of the invention to provide a circuit configuration to implement the novel process.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of converting a frequency signal to a DC voltage signal, which comprises:
inputting a frequency signal having a given frequency;
generating a first and a second output voltage signal from the frequency signal, each formed of a series of rectangular pulses with a pulse sequence frequency equal to the given frequency (f) of the frequency signal and a pulse width;
converting the first output voltage signal with a first lowpass filter to a first DC voltage signal, and converting the second output voltage signals with a second lowpass filter to a second DC voltage signal; and
influencing with the second DC voltage signal a pulse width of the rectangular pulses of at least the first output voltage signal.
In other words, in the process for converting a frequency signal to a DC voltage, a first and second output voltage signal are generated from the frequency signal, each of which consists of a sequence of rectangular pulses, the pulse sequence frequency (the period) of which is equal to the frequency of the frequency signal. With a first respectively a second lowpass filter, the first and second output voltage signal are converted to a first respectively a second DC voltage signal. The second DC voltage signal is used to influence the pulse width of the rectangular pulse of at least the first output voltage signal.
As a consequence, it is possible to change the magnitude or value of the first DC voltage signal as a function of the frequency of the frequency signal and to compensate linearity errors. For example, if the pulse width is increased, the magnitude of the first DC voltage signal available at the output of the first lowpass filter also increases while the frequency remains constant. This makes it possible to compensate a linearity error with negative sign occurring at constant pulse width, i.e., the value of the uncompensated DC voltage signal would correspond to a smaller frequency than would be the case with exact linearity. Depending on the sign of the linearity error, the pulse width of the first output voltage signal is thus either increased or decreased.
In accordance with an added feature of the invention, the second output voltage signal is generated by inverting the first output voltage signal. The first DC voltage signal and the second DC voltage signal are thus opposed to each other. With the use of common monoflop ICs, this permits particularly simple compensation of the linearity error, which in this case is typically negative. In this case, the second DC voltage signal also influences the pulse width of the second output voltage signal, since the first and second output voltage signals are merely inverted with respect to each other.
With the above and other objects in view there is also provided, in accordance with the invention, a circuit configuration for converting a frequency signal to a DC voltage, comprising:
a circuit having an input receiving a frequency signal having a given frequency, an output outputting a first output voltage signal and a second output voltage signal, and a control input, the circuit generating the first and second output voltage signals from the frequency signal each with a sequence of rectangular pulses having a pulse sequence frequency equal to the given frequency of the frequency signal;
a first lowpass filter connected to the output for converting the first output voltage signal to a first voltage signal; and
a second lowpass filter connected to the output for converting the second output voltage signal to a second voltage signal, and connected to the control input of the circuit for determining a pulse width T0 of the rectangular pulses of the first output voltage signal.
In accordance with another feature of the invention, the circuit is a monostable trigger circuit. This permits the use of cost-effective standard monoflops of the CMOS or TTL series.
In accordance with a further feature of the invention, the second output voltage signal is inverted relative to the first output voltage signal. This permits particularly simple compensation of the linearity error, which is typically negative in standard monoflops.
In accordance with again an added feature of the invention, the monostable trigger circuit has a first output for outputting the first output voltage signal and a second output for outputting the second output voltage signal. The two output signals of the monostable trigger circuit are thereby inverted relative to one another.
In accordance with a concomitant feature of the invention, an ohmic feedback resistor is connected between the output of the second lowpass filter and the control input of the monostable trigger circuit. The value of the ohmic feedback resistor must be selected such that the linearity error becomes as small as possible within the designated output voltage range.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in process and circuit configuration for converting a frequency signal to a DC voltage, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.